This invention relates to thin film solid state electrochemical cells. More particularly, this invention relates to a novel positive electrode material having the formula (CS.sub.x).sub.n wherein x is from about 1.2 to about 2.3 and n is unspecified but at least greater or equal to 2.
Batteries are used in almost all consumer electronic products from flash lights to lap top computers. Over the years, considerable interest has been shown in developing high energy-density rechargeable batteries for many applications including electric vehicles. In this regard, thin film solid state batteries using the organo-sulfur cathode materials of this invention are particularly well suited for use in many consumer applications because of their high energy to weight ratio.
Two main types of cathode materials used in the manufacture of thin film lithium and sodium batteries are known in the art. The first materials include transition metal chalcogenides, such as titanium disulfide with alkali metals as the anode. For example, among the cathode active chalcogenides, U.S. Pat. No. 4,049,879 lists transition metal phosphorous chalcogenides. Other U.S. patents, such as U.S. Pat. Nos. 4,143,214, 4,152,491 and 4,664,991 describes cells wherein the cathode is a carbon/sulfur based material, generally of the C.sub.x S formula where x is typically 10 or larger.
U.S. Pat. No. 4,143,294 to Chang, et al. describes cells having cathodes containing C.sub.x S, wherein x is a numerical value from about 4 to about 50. U.S. Pat. No. 4,152,491 to Chang, et al. relates to electric current producing cells where the cathode-active materials include one or more polymer compounds having a plurality of carbon monosulfide units. The carbon monosulfide unit is generally described as (CS).sub.x, wherein x is an integer of at least 5, and may be at least 50, and is preferably at least 100. In both cells developed by Chang, et al. the energy storage capacity is limited because there is a low density of sulfur-sulfur bonds.
U.S. Pat. No. 4,664,991 to Perichaud, et al. describes a substance containing a one-dimensional electric conducting polymer and at least one polysulfurated chain forming a complex with the polymer. Perichaud, et al. use a material which has two components. One is the conducting polymer, which is selected from a group consisting of polyacetylenes, polyparaphenylenes, polythiophenes, polypyrroles, polyanilines and their substituted derivatives. The other is a polysulfurated chain which is in a charge transfer relation to the conducting polymer. The polysulfurated chain is formed by high temperature heating of sulfur to produce a chain of the composition . . . S--S--S--S . . . of indeterminate length. As a result of using this material, the cell of Perichaud, et at. exhibits a fairly low voltage of only 2.0 V against lithium.
In a related approach, U.S. Pat. Nos. 4,833,048 and 4,917,974 to De Jonghe, et al. describe a class of cathode materials made of organo-sulfur compounds of the formula (R(S).sub.y).sub.n where y=1 to 6; n=2 to 20, and R is one or more different aliphatic or aromatic organic moieties having one to twenty carbon atoms. One or more oxygen, sulfur, nitrogen or fluorine atoms associated with the chain when R is an aliphatic chain can also be included. The aliphatic chain may be linear or branched, saturated or unsaturated. The aliphatic chain or the aromatic rings may have substituted groups. The preferred form of the cathode material is a simple dimer or (RS).sub.2. When the organic moiety R is a straight or a branched aliphatic chain, such moieties as alkyl, alkenyl, alkynyl, alkoxyalkyl, alkythioalkyl, or aminoalkyl group and their fluorine derivatives may be included. When the organic moiety comprises an aromatic group, the group may comprise an aryl, aralkyl or alkylaryl, including fluorine substituted derivatives, and the ring may also contain one or more nitrogen, sulfur, or oxygen heteroatoms in the ring as well.
In the cell developed by De Jonghe, et al. the main cathode reaction during discharge and recharge of the battery, is the breaking and reforming of S--S bonds. The breaking of an S--S bond is associated with the formation of an S.sup.- -M.sup.+ ionic complex. The organo-sulfur materials investigated by De Jonghe, et al. undergo polymerization and de-polymerization upon the formation and breaking of the S--S bonds. The de-polymerization which occurs during the discharging of the cell results in lower weight monomeric species which can dissolve into the electrolyte, severely reducing the utility of the organo-sulfur material as cathode-active material. The result is an unsatisfactory cycle life having a maximum of about 200 deep discharge-charge cycles, more typically less than 100 cycles as described in J. Electrochem. Soc., Vol 138, pp. 1891-1895 (1991). In particular, the organo-sulfur materials developed by De Jonghe, et al., are highly unstable in the presence of high conductivity plasticized, or gel electrolytes.
A significant additional drawback with the organo-sulfur materials developed by De Jonghe, et al. is the slow kinetics of oxidation and reduction at ambient temperatures, severely reducing the power of cells incorporating cathodes made with the organo-sulfur materials as cathode-active materials. The slow kinetics is due to the oxidation and reduction being related to the formation and breaking of sulfur-sulfur bonds, respectively. In contrast, the organo-sulfur materials of the present invention are conjugated structures where the oxidation and reduction are independent of S--S bond formation and breaking and maintains a fast kinetic rate at ambient temperatures and below.
Despite the various approaches proposed for organo-sulfur cathode materials, there remains a need for an inexpensive cathode material having a high storage capacity, high discharge rate and a very long cycle life at ambient and sub-ambient temperatures.
It is, therefore, a primary object of this invention to provide new organo-sulfur based cathode materials for thin film solid state batteries which are inexpensive, yet avoid the limitations existing in the prior art, while offering performance characteristics much higher than those of known materials having an actual specific energy in excess of 900 Wh/kg.
It is another object of this invention to provide a new cathode having as the active cathode material poly(carbon disulfide) (PCS) polymers which does not undergo polymerization and de-polymerization upon oxidation and reduction.
It is yet another object of this invention to provide a method of making a solid state rechargeable battery including the novel cathode of the invention.